A pedigreed colony of the Chilean hystricognathous rodent Octodon degus, has been established. The animals are maintained in a closed, random bred colony; in addition, an inbreeding program is in progress to derive isogeneic degus. The degu is a rat-like rodent reaching an adult weight of approximately 300 grams. The coat color is agouti; a coat color variant has been identified. This species belongs to the porcupine suborder, and is related at the subordinal level to the guinea pig. Gestation time is 90 days, with two apparently seasonal breeding periods per year. The animal is docile and thrives in conventional vivarium facilities with no extraordinary care required. The degu has an anatomically separate dual thymus system: a large, bilateral cervical thymus lying caudal to the submaxillary salivary gland in the neck; and a mediastinal thymus. Spontaneous bilateral cataracts arise with considerable frequency, and cataractous degus appear to be hyperglycemic. Immunological studies include: 1. Allogeneic skin transplants in new born degus; studies of the frequency, morphology, and distribution of antigen-binding cells in cervical and mediastinal thymus, peripheral lymphoid tissues, and buffy coat cells using beta-galactosidase as antigen, and the chromogenic substrate, 5-bromo- 4-chloro-3-indolyl-beta-D-galactoside; the effect of immunization on decreasing cell frequencies of antigen-binding cells specific for galactosidase in thymic tissue; the effect of cortisone on antigen-binding cell frequencies; the ability of affinity-labeled galactosidase to specifically inhibit antigen-binding.